Generally, for painting broad surface areas of a work piece such as vehicle body, furniture and electric appliance, it is the usual practice to use a rotary atomizing head type coating apparatus which can cope with large spray patterns. On the other hand, for painting small surfaces with minute ups and downs, it is often the case to resort to a pneumatic atomization or air atomizing type coating apparatus (an air spray gun) which is capable of spraying paint particles with high directionality in a straightforward direction.
An air atomizing type coating apparatus of this sort is largely constituted by a main body to be attached on a fore end portion of an arm of a coating robot or to be held in an operator's hand, an air atomizing type paint atomizing means attached to the front side of the main body and equipped a paint nozzle which spurts paint in combination with an air nozzle which is adapted to spurt jets of atomizing air to atomize paint which is being spouted out from the paint nozzle, and a paint passage for circulation of paint toward the paint atomizing means (see, for example, Patent Literature 1: Japanese Patent Laid-Open No. H1-317563).
At the time of a coating operation, the coating apparatus is connected to a color changing valve unit through a paint feed pipe, supplying paint to the paint atomizing means by way of the paint feed pipe. By atomizing air, paint is sprayed forward from the paint atomizing means in a finely divided form for deposition on a work piece. On the other hand, at the time of changing the color of paint, paint deposited on the paint feed pipe as well as the paint atomizing means are washed with wash fluids after discharging residues of a previous color from the paint feed pipe.
On the other hand, it has been known to construct an air atomizing type coating apparatus as an electrostatic coating apparatus, which is arranged to apply a high voltage to conducting paint for the purpose of enhancing paint deposition efficiency against a work piece, putting paint particles on a flight along lines of electric force which are formed between the electrostatic coating apparatus and the work piece (see, for example, Patent Literature 2: Japanese Patent Laid-Open No. H7-194998).
In this regard, in case a high voltage is applied directly to a conducting paint which is aqueous paint or metallic paint in circulation through a paint feed pipe, leakage of a high voltage to the side of earth ground takes place through paint in the paint feed pipe. Therefore, in the case of the air atomizing type coating apparatus of Patent Literature 2 mentioned above, an electrode is attached to a nose end of the paint atomizing means to form a corona discharge region forward of the coating apparatus, charging sprayed paint particles with a high voltage indirectly while they pass through the corona discharge region.
In the case of the air atomizing type coating apparatus of above-mentioned Patent Literature 1, the paint color can be changed to a new color after discharging residues of a previous color from an elongated paint feed pipe and washing away internally deposited paint from the entire length of the paint feed pipe and from the paint atomizing means as well by using a wash fluid. That is to say, on a color change, a large amount of paint has to be wastefully discarded, in addition to the necessity for a time-consuming washing operation involving consumption of large amounts of wash fluids.
Besides, in the case of the air atomizing type coating apparatus of above-mentioned Patent Literature 2, a corona discharge region is formed by an electrode which is attached to the nose end of the paint atomizing means thereby to indirectly charge paint particles with a high voltage. However, as compared with a direct charging coating apparatus, an indirect charging coating apparatus is more difficult to charge all of sprayed paint particles with a high voltage in a stable state, and thus has problems such as low paint deposition efficiency and inferior finish quality of coatings.